Where to leave Rental Car close to London?

St. Albans would be a good place to drop off car, then you can take the train into London.

St. Albans is a good town to spend a few hours. There is the abbey church, and Verulamium Roman town (just the foundations left). Note the main station is a good 15 minute walk east of the town centre.

Another "drop off and spend half a day" option is Cambridge. Plenty of trains to London.

The Bayswater neighborhood is one of my favorites to stay in because public transport is so convenient with two tube stops and great bus routes along Bayswater and Westbourne roads. Its close to Paddington station and an easy stroll into and through Hyde Park and Kensington Gardens. Same for enjoying the nearby Portobello Road Market and the adjacent Holland Park and Nottinghill Neighborhoods. Are you looking for another neighborhood option? What month are you traveling? What are you looking to see and do on this visit? Happy to suggest places to stay but a little unclear what you seek as far as a neighborhood?

I'm not sure what level of service you're looking for in a B&B, but I recently stayed in the Fruit Guest House and liked it a lot:

https://www.bedandbreakfast.eu/bed-and-breakfast/london/fruit-guest-house/1110953/

It's a pretty basic place and not luxurious by any means, but the location (very close to the Royal Oak tube station) and price (around $70 US for a double for the nights we stayed, although that was in February and they seem to be raising their rates for the summer) are both excellent.

I am wondering about High Barnet. It is at the end of the Northern Line tube, is a fairly large place, and will certainly be on the way from Scotland. The ride on the Underground to central London is about an hour.

Thank you for all the choices, I guess I should clarify we are going to stay west, from Skye down into Wales (husband's origins) then over to south west, Devon & Cornwall. We don't have anything booked yet, including flight home.. Bayswater was great, but I had hoped for advice on a B &B further east because it seemed like we were far away from everything. We didn't walk the streets much, saw Westminster, Tower, Churchill war rooms..everything in Greenwich. not looking for a fancy B & B, but not a hostel! We've been advised not to do the bus because traffic's so bad. Does the above-ground train go into downtown? Sorry to be so hopeless this is overwhelming even tho we've been there!

Tom, "Does the above-ground train go into downtown?" - Train from where? you don't say.
Trains from anywhere (commuter, overground, Inter-City) terminate at large stations on the edge of central London, in most cases on the circle underground. You then get the tube to your final destination.
London doesn't have a "downtown". Nowhere in England does, it is not an English word. London has two centres because it was once two separate cities, like New York and Washington.

• "The city", the business district on the site of the old medieval city. With St Pauls, the tower etc.
• "Westminster", the government district, with the House of Parliament, Whitehall, No. 10, Buck House etc.

The area in between is the "West end" (west of the city), this includes Bayswater, Soho etc.

Thank you everybody for your thoughtful answers. I'm liking the idea of Cambridge, but now at least I have a list! Obviously I should have booked rooms a long time ago!

Thank you everybody for your thoughtful answers. I'm liking the idea of Cambridge, but now at least I have a list! Obviously I should have booked rooms a long time ago!